One of the most critical aspects of the control of the operation of railway vehicles, particularly freight trains, is the predictable successful operation of the air brake system. The air brake system is subjected to a variety of dynamic effects, not only as a result of the controlled application and release of brake pipe pressure in response to varying conditions encountered by the train, but as a result of the occurrence of unpredictable anomalies in the integrity of the brake system itself.
More particularly, with reference to FIG. 1, which diagrammatically illustrates a typical air brake system employed by a railway freight train, the application and release of braking action is generally controlled by the engineman within locomotive equipment 10. Locomotive 10 contains an air brake control system including a controllably pressurized brake pipe 101, which is coupled (via one of a series of cut-out valves 120) to the serial train air line 201, through which air brake pressure is supplied for each of the cars 20 of the train. The air brake control system also includes an air supply input link 111 for supplying, under pressure, fluid (air) through which the brake pipe and train air line are charged, for controlling the operation of the pneumatically operated brake mechanisms at the wheels of the train.
Coupled within air supply input link 111, for measuring the charging rate (in terms of air supply pressure) of the brake pipe supply air are an air flow measuring adaptor 113 and an associated gauge 115. Air supply link 111 is coupled to a first, input port 121 of a relay valve 117 and a second, bidirectional port 122 of which is coupled to the brake pipe 101. Relay valve further includes a third port 123 which is coupled through an air pressure control link 103 to an equalizing reservoir 105 and an equalizing reservoir pressure control unit 107 through which the line 103 and equalizing reservoir 105 are controllably charged and discharged in the course of a braking operation. A fourth port 124 of relay valve 117 is controllably vented to the atmosphere as an exhaust port. Coupled with brake pipe 101 and air pressure control link 103 are respective pressure measuring gauges 131 and 133 through which the pressure in the brake pipe 101 and that in the air pressure control link 103 are monitored.
The brake control unit within a typical car 20 of the train includes a control valve 203, a first port 221 of which is coupled to the train air line 201. Control valve 203 has a second port 222 coupled to pressure storage and reference reservoir(s) 205, and a third port 223 which is coupled to the actual brake cylinder 231 that controls the movement of the brake shoes 233 relative to the wheels 235 of the car.
In operation, the cut-out valve 120, through which the brake pipe 101 and successive segments of the train air line 201 are coupled in serial fluid communication, are assumed to be fully open, so that there will be a continuous brake/air line fluid path between the locomotive equipment 10 and all of the cars 20 of the train. The brake system is initially pressurized by the operation of pressure control unit 107, which couples the air supply to line 103, so as to fully charge equalizing reservoir 105. Relay valve 117 is then operated to couple port 121 with port 122, so that air is supplied therethrough to brake pipe 101 and thereby to the train air line 201, in order to charge the brake pipe/air line fluid path 101/201 to its maximum charging pressure, as established by the pressure (typically 85 psi.) of equalizing reservoir 105 within locomotive equipment 10. Namely, the pressure within the brake pipe 101 (and train air line 201) is determined to have reached the maximum, as established by the fully charged equalizing reservoir 105, when the pressure at port 122 matches the pressure at port 123. Through control valves 203 in each of the cars 20 of the train, pressure storage and reference reservoirs 205 are fully charged, to thereby establish a reference pressure for maximum withdrawal of the piston of each brake cylinder 231 and thereby complete release of the brakes for each of the cars 20.
When the engineman desires to apply brakes to the wheels of the train cars, he operates pressure control unit 107, typically via a handle-operated pneumatic control valve, which is coupled to the air pressure control link 103. Operation of pressure control unit 107 will cause a partial venting of air pressure control link 103 and thereby a reduction in the pressure within equalizing reservoir 105 (through link 103). This reduction in pressure in the equalizing reservoir 105 is sensed by relay valve 117 (via port 123) which, in turn, causes its bidirectional port 122 to be coupled to exhaust port 124 and thereby vent the brake pipe 101 to the atmosphere until the brake pipe pressure equals the pressure of equalizing reservoir 105. Because of the considerably larger volume of the fluid path through the brake pipe/train air line linkage 101/201, the length of time required for air pressure within the brake pipe/air line to drop to the pressure in the equalizing reservoir 105 is significantly longer than the time required for a reduction in air pressure in the equalizing reservoir 105, which takes place quickly in response to the operation of pressure control unit 107.
As the pressure in the brake pipe 101 and, correspondingly, within train air line 201, drops, the respective control valves 203 in each of the cars 20 senses the pressure reduction in the train air line 201 and causes a corresponding increase in the pressure applied to the brake cylinders 231, resulting in an application of brakes to the wheels in proportion to the sensed pressure reduction in train air line 201. Further pressure reductions in the equalizing reservoir 105 by the engineman produce corresponding pressure reductions in the train air line 201 and, thereby, additional braking effort by the brake mechanisms in each of the cars 20. In other words, for the intended operation of the brake system, the braking effort applied in each of the cars is proportional to the reduction in pressure in the equalizing reservoir 105 within the locomotive equipment 10.
When the engineman desires to release the brakes, he operates pressure control unit 107 to effect a recharging of the system, to bring the pressure within equalizing reservoir back up to its fully charged state, as described previously. With equalizing reservoir 105 recharged, there is again a pressure differential (but opposite to the previous drop in the pressure in line 103) between ports 122 and 123, which is sensed by relay valve 117. Valve 117 thereby couples air supply link 111 to brake pipe 101, so as to recharge and thereby increase the pressure in the brake pipe/train air line 101/201. This increase in pressure within the train air line 101/201 is sensed by the control values 203 in each of cars 20, so as to cause the brakes to be released.
During normal operation, the application and release of brakes is controlled in accordance with the above-described sequence of events. However, there may be circumstances, dictated either by action taken by the engineman or by other unpredictable events, which create the potential for unsafe operation of the braking system. One of these conditions occurs as a result of the engineman applying braking subsequent to the release of a previous brake application, but prior to the system having been recharged and pressure within the brake pipe/air line fluid path stabilized.
More specifically, when the engineman initiates a release of the brakes of the train after a previous braking application, air for charging the brakes system is input through the air supply link 111 and relay valve 117, so as to recharge the brake pipe 101 and the train air line 201, as pointed out above. During this brake release/air line charging interval, the pressure within the brake pipe 101 rises slowly (compared with the rapid rate of charge of equalizing reservoir 105, which has been recharged by the operation of pressure control unit 107) until it equals the pressure of link 103, at which time relay valve 117 interrupts the connection of the air supply link 111 to the brake pipe 101, as the train air line 201 is now fully charged.
If, prior to the brake pipe 101 and train air line 201 becoming fully charged, the engineman initiates a new brake application (by reducing the pressure in the equalizing reservoir 105 via operation of pressure control unit 107), the pressure differential between the partially charged train air line 201 and pressure storage reservoirs 205 in each of cars 20 will be different than the pressure differential intended by the pressure reduction newly applied to equalizing reservoir 105. As a consequence, each of control valves 203 will sense a smaller pressure differential between ports 221 and 222 than the drop applied to equalizing reservoir 105, so that the braking effort imparted to brake cylinders 231 in each of cars 20 will be less than what the engineman has requested. If not immediately recognized by the engineman, this "reduced-effort" braking application can create a potentially unsafe condition of the train.
Still, even when he recognizes the insufficiency of the new braking application, the engineman often attempts to remedy the problem by a further incremental reduction in the pressure in the equalizing reservoir 105. Again, however, the application of only a partial braking effort described above will take place, so that there may still be inadequate braking action applied by the cars of the train. Simply put, if the engineman tries to make up for insufficient braking of one pressure reduction request in a piecemeal fashion, and fails each time, it is possible that continuing efforts in this process will be unsuccessful and the originally intended braking effort will never be accomplished.
An experienced engineman (in terms of the train and conditions the train currently encounters), upon realizing the unsafe condition, may apply a severe pressure reduction in an effort to make up for the original lack of braking response to the pressure reduction of an incompletely charged system. Still, the action taken by the engineman is only a guess; a guess (even an educated one) as to whether a further braking effort will successfully brake the train cannot be equated with safe train operation.
Another circumstance in which a `guesstimate` braking control procedure is used is in the course of determining when the train air line is fully charged, so that the brakes are fully released and a motive traction force can be applied to the train. For this purpose, a practice commonly employed by the engineman is his interpretation of the sound of air passing through the brake relay valve to determine when the brakes are fully released or when the train air line is fully charged. In effect, this practice constitutes a "seat of the pants" procedure which is not necessarily reliable.
As pointed out above, in addition to potential safety hazards that arise as the result of the application of insufficient brake control pressure by the engineman (due, for example, to an unexpected dynamic event), the integrity of the train air line is subject to unforeseeable changes (such as a cut out valve being accidentally hit or tampered with, resulting in a change in air line continuity) which, if remained undetected, could cause the brakes to be applied to one part of the train but not applied or only partially applied to another part of the train. The need for qualifying the integrity of the air line is especially important when the engineman proceeds to cut the engine off from the rest of the train at a destination or for switching purposes. In the course of this procedure, the engineman will normally initiate a full brake application on the train, so as to lock the wheels of the cars. After applying a "full brakes" pressure input to the pressure control unit 107, the enginemen will wait for some period of time until the brake pressure application has propagated down the entire train and the air exhaust port 123 of relay valve 117 has ceased venting brake pipe pressure to the atmosphere. Once the engineman is satisfied that the air pressure has been completed released by listening to what he considers to be the last venting of air from the exhaust port of relay valve 117, he then proceeds to disconnect or close the brake pipe-train air line cut-off valve 120 and uncouple the locomotive(s) from the rest of the train.
If the pressure in the train air line has not stabilized prior to the engineman cutting off the engine from the rest of the train, an undesired release of the brakes on the cars may be caused by an abrupt change in the air flow from the train air line. In addition, if there is an obstruction in the line, preventing proper venting of brake pipe/air line pressure, the brakes of some or all of the cars of the train may actually be released! Again, the enginemen's reliance upon what he hears as a control criterion for the operation of a freight train is far from a reasonably safe railway operation procedure.